Multi-source active injection load pull system and method

ABSTRACT

A harmonic load pull system uses a number of synchronized signal sources to inject harmonic power into the output of a power transistor. The mismatch between the signal sources and the transistor internal impedance is reduced by using multi-harmonic tuners, which pre-match the impedance of the signal sources to the internal impedance of the transistor at each harmonic frequency independently. Actual tuning is electronic by changing the amplitude and phase of the injected signals. The transmitted and reflected waves are measured and optimized through bi-directional couplers connected between the transistor and the tuner and a wideband harmonic receiver.

PRIORITY CLAIM

This application claims priority on provisional application 61/660,158filed on Jun. 15, 2012.

CROSS-REFERENCE TO RELATED ARTICLES

-   -   [1] “ALPS-308, Active Load Pull System for PCN Applications”,        Product Note 33, Focus Microwaves Inc., April 1996.    -   [2] “MPT a universal Multi-Purpose Tuner”, Product Note 79,        Focus Microwaves Inc., October 2004.    -   [3] POWER AMPLIFIERS:        http://www.amlj.com/display2.php3?FreqRange=&freqband=ganamps&location=cam    -   [4] Z. ABOUSH et al., “High power active harmonic load-pull        system for characterization of high power 100 Watt transistor”,        35^(th) EuMC, October 2005, Paris.    -   [5] “Cascading Tuners For High-VSWR And Harmonic Load Pull”,        Maury Microwave Application Note 5C-081, January 2009.    -   [6] “Hybrid Active and Harmonic Tuning”, Application Note 62,        Focus Microwaves Inc., December 2011.    -   [7] U.S. Pat. No. 5,276,411, High power solid state programmable        load.

BACKGROUND OF THE INVENTION Prior Art

RF and Microwave transistor chips are best characterized “on wafer”.This allows avoidance of parasitic connection elements, like wire bondsand fringe capacitors, which are associated with packaging the devicesin order to mount them in test fixtures. It also allows a much largernumber of devices to be tested “in situ” without having to laboriouslyslice the wafer, mount and wire-bond each individual chip. The “onwafer” testing is at this time the preferred testing method, except forvery high power devices, beyond 10 Watt RF power. On-wafer testing isalso the exclusive testing method in millimeterwave frequencies, sincedevice packaging is extremely difficult and the parasitic elementsassociated with the package (inductance of wire bonds and fringecapacitors of package housings) would falsify the measured data to thepoint of uselessness.

However, on-wafer testing implies reduced tuning range. This is becauseof the insertion loss of the connection between the tuner and the waferprobes (11) and the insertion loss of the probes themselves (FIG. 1).The DUT, including wafer probes or embedded in a test fixture, isrepresented schematically in box (12). To compensate for the saidinsertion loss active load pull has been introduced [1]. The latestcomprehensive active load pull technique is the so called “activeinjection” load pull (FIG. 1). In this setup a harmonic receiver (13) isused which also provides two or more coherent signal sources (14, 15);coherent means the signals have a fixed controlled phase between them.In this configuration signal (15) being injected into the output (11) ofthe DUT is at the same frequency and has its phase controlledelectronically relative to the phase of signal (14). The signal (14) isinjected into the input of the DUT through an (optional) driveramplifier (not shown) and an impedance tuner (16) and an inputdirectional coupler (17). The coupler (17) feeds (18) a small part ofthe injected (a1) and reflected (b1) signal waves into the harmonicreceiver (13). The signal going out of the DUT (11) passes throughanother directional coupler (19), which also feeds a small part of theinjected (a2) and reflected (b2) signal waves into the harmonic receiver(13), and said outgoing signal (11) interacts with the signal injectedfrom the power amplifier (111). Through amplitude and phase control ofthe injected signal (15) a virtual reflection factor Γ is generated atthe output (11) of the DUT (12): Γ=a2/b2. Since the amplifier (111) canincrease the injected signal amplitude at will, the power returning fromthe load (a2) can be made larger than the power leaving the DUT (b2). Inpractical terms this means a reflection factor Γ>1. This allows matchingany internal impedance (reflection factor) of the DUT despite anyinsertion losses.

The relation between reflection factor and impedance is:Z=Zo*(1+Γ)/(1−Γ); where Zo is a standard impedance (typically 50 Ohm);this means that for Γ=−1, Z=0 Ohm.

The actual problem with this configuration is that there exist,typically, a large impedance mismatch between the output of the poweramplifier (111) and the output of the DUT (11). Typical impedances ofpower transistors (DUT) is 0.5-2 Ohms. Typical impedance of poweramplifiers is 50 Ohms. This creates a large mismatch ratio between 25:1and 100:1. The consequence is that the power required from the amplifier(111) is typically 20 times larger or more than the power generated bythe DUT [4]. This requirement can be reduced if a transformer is usedbetween the DUT (12) and the amplifier (111). This is shown in FIG. 2.The impedance Tuner (21) transforms the (typically) 50 Ohm outputimpedance of the amplifier (22, 111) closer to the (typically 0.5-2Ohms) output impedance of the DUT (23). The mismatch reduction(improvement) factor can reach high values, depending on the impedancethe tuner (21) can generate and present to the DUT (23). The mismatchfactor can be expressed as a “standing wave voltage ratio VSWR”; in asystem with a characteristic impedance of 50 Ohm VSWR=50Ω/R.dut, whereR.dut is the internal output impedance of the DUT (typically 0.5-2Ohms). This gives an idea of the actual mismatch: VSWR is between 25 and100. A transformer (such a tuner) will create, typically, a pre-matchfactor of the order of 5:1 to 15:1. Knowing that the total mismatch isthe product of the partial mismatches, in this case the total mismatchwill be reduced by this factor; a 25:1 initial mismatch will be reducedby the said tuner/transformer to values between 5:1 and 1.7:1 (=25/15:1)and an initial mismatch of 100:1 will be reduced to values between 20:1and 6.7:1. If the tuner can actually reach exactly the conjugateimpedance of the DUT (tuner VSWR=DUT VSWR) then no additional signalpower (24) will be needed [4]. If it can only reach values close to theconjugate internal impedance of the DUT then only a reduced signal power(24) is required [2]. This reduces significantly the complexity andespecially the cost of the test setup, since power amplifiers can beexpensive.

Harmonic tuning, i.e. independent impedance control at the harmonicfrequencies can be materialized using two or more signals (31, 32)injected into a frequency combiner (35) and then into the output of theDUT (FIGS. 3 and 7). These signals must be coherent with the inputsignal at frequency Fo (34), whereas one signal (31) is at the same,fundamental, frequency (Fo) and the other (32) is typically at the firstor even at a higher harmonic frequency (2Fo or 3Fo etc. . . . ). Theproblem with the setup of FIG. 3 is that a wideband impedance tuner (33)is used. A wideband tuner creates reflection over a large frequencyrange, which, typically, includes the fundamental and several harmonicfrequencies. Whereas controllable impedances are created only at one(typically the fundamental, frequency Fo) uncontrollable reflections arecreated at any other frequency. It may therefore happen that thereflection factors shown to the DUT at a harmonic is slightly orradically different than the internal impedance of the DUT at thisfrequency (FIG. 4). The dots in FIG. 4 show a scenario where theuncontrollable reflections at the harmonic frequencies (2Fo and 3Fo) areanti-diametric (180 degrees off) to the optimum reflection factors ofthe DUT at said frequencies. This is not the usual scenario, but apossible one, that an efficient test setup should be able to handleroutinely.

In FIG. 4 the dot marked “Fo” is noted as “reduced because of couplerloss”: this means that a typical setup as discussed here (FIGS. 1, 2, 3)includes directional couplers adjacent to the DUT (17, 19) in order todetect the incident and reflected waves by the receiver (13). In such acase the insertion loss in said couplers will reduce the tuning range ofsaid tuner (21). This explains the markings on “Fo” in FIG. 4.

The effect of harmonic impedance mismatch (FIG. 4) is shownquantitatively in FIGS. 5 and 6: the plot in FIG. 5 represents acalculation of injected power required to match the harmonic impedanceof a DUT as a function of the phase mismatch between the reflectionfactor generated by the wideband tuner (33) and the DUT. The actualnumbers shown are for a typical 20 Watt DUT operated at mediumcompression and generating harmonic power at 2Fo of 0.4 Watt(P(2Fo)/P(Fo)=−17 dB). The plot is typical for many practical cases. Itshows at point (52) that, if the mismatch angle is 180 degrees therequirement for harmonic injection power increases by a factor of 33.75(13.5/0.4) relative to the case where proper pre-matching reduces theinjection power requirement is minimum or even zero, as shown at point(51). A similar calculation holds for 3Fo as well. Because harmonicfrequencies are high, associated power amplifiers are very expensive[3]. This is all in addition to any unavoidable connection and adapterlosses. Therefore an appropriate means for reducing said powerrequirement will reduce sensibly the cost of the setup.

DESCRIPTION OF THE DRAWINGS

The disclosed invention will be better understood when viewed togetherwith the enclosed pictures, as follows:

FIG. 1 depicts prior art: the block diagram of an open loop activeinjection load pull test setup.

FIG. 2 depicts prior art: the block diagram of an open loop activeinjection load pull test setup using a wideband pre-matching impedancetuner.

FIG. 3 depicts prior art: the block diagram of an open loop activeinjection load pull test setup using two synchronized sources (typicallyFo (fundamental) and 2Fo (second harmonic)) and a wideband pre-matchingimpedance tuner.

FIG. 4 depicts prior art: possible distribution of harmonic impedances(reflection factors) of a wideband impedance tuner on a Smith chart andassociated optimum impedances to be presented to the DUT, showingpossible significant differences.

FIG. 5 depicts prior art: Injection power requirement to match theoutput impedance of a DUT as a function of the phase difference betweenDUT (target) and reflection factors created by a wideband tuner (comparewith FIG. 4).

FIG. 6 depicts prior art: possible (close to “worst case”) tuningmismatch situations when using a wideband impedance tuner.

FIG. 7 depicts an open loop active injection load pull test setup usingtwo synchronized signal sources (typically Fo and 2Fo) and a two probetwo-frequency (harmonic) impedance tuner.

FIG. 8 depicts an open loop active injection load pull test setup usingthree synchronized signal sources (typically harmonics Fo, 2Fo, 3Fo) anda three probe three-frequency (harmonic) impedance tuner.

FIG. 9 depicts an open loop active injection load pull test setup usingtwo synchronized signal sources (typically Fo and 2Fo) and a cascade oftwo single probe, wideband impedance tuners.

FIG. 10 depicts an open loop active injection load pull using threesynchronized signal sources (typically Fo, 2Fo and 3Fo) and a cascade ofsingle and double probe impedance tuners.

FIG. 11 depicts the reduction effect of passive prematching on injectionpower requirements as a function of the internal impedance of the DUT ina 50 Ohm test system. The vertical axis (113) shows the requiredinjected power (Pinj) as related to the DUT output power (Pout); thescale range is 0 to 1400%.

FIG. 12 depicts the creation of overall reflection factors (121, 122 and123) at DUT output port as a combination of passive reflection factors(124, 125 and 126) and active, “power-injection created”, reflectionfactors (127, 128 and 129) for three harmonic frequencies (Fo, 2Fo and3Fo); the same scheme is valid for any (not harmonic) frequency as well.

DETAILED DESCRIPTION OF THE INVENTION

The proposed solution serving to reduce the power requirement of theinjection amplifiers in open loop harmonic injection load pull testsystems is shown in FIGS. 7 to 9 [6]. Instead of using single carriagewideband tuners (33) another type of impedance tuners are employed, themulti-carriage harmonic tuners [2] (71, 81). Harmonic tuners [2] operatein such a way as to be able to generate arbitrary impedances Z(Fi) atdifferent frequencies, including harmonic frequencies Fi (Fo, 2Fo, 3Fo .. . ), independently on each-other. The harmonic tuners in the setups ofFIGS. 7 to 9 allow the pre-matching of the DUT to the load at eachharmonic frequency independently and as close as possible to theconjugate complex internal impedance (optimum matching impedance) ofsaid DUT at the specific frequency. This allows operating at point (51)in the diagram of FIG. 5, i.e. with minimum or zero requirement ofinjected power at the fundamental and any harmonic frequency. It has tobe emphasized that the relationship in FIG. 5 is valid for anyfrequency, including the fundamental (Fo) and all harmonics (2Fo, 3Fo, .. . ). Alternatively to single housing harmonic tuners the cascade oftwo or more wideband tuners can be used to perform harmonic tuning (91,92), though with less efficiency.

In the setup of FIG. 7 a two-probe two-frequency harmonic tuner (71) isrequired, since the only controlled second harmonic source is at 2Fo(72). In this case the impedance at the harmonic frequency 3Fo isignored and no power is injected at this frequency. The same setup couldbe configured to control Fo and 3Fo and ignore the second harmonic 2Fo;this depends on the nonlinear mode of operation of the DUT and theactual waveform of voltages and currents through the DUT and theassociated Fourier harmonic frequency components.

In FIG. 8 the fundamental (Fo) and two harmonic frequencies (2Fo, 3Fo)are processed. The fundamental frequency is injected through thereceiver's (VNA's) second internal source (82) the second harmonicthrough a first external source (83) and the third harmonic (3Fo)through a second external source (84). All three frequencies arecombined through the frequency/power combiner (85) and injected into themulti-probe (harmonic) tuner (81). Said tuner is a three-probe tuner [2]which can tune all three frequencies independently, thus reducing thepower requirement from the sources (82, 83, 84) and the output power ofthe associated power amplifiers (86, 87, 88).

A similar configuration as in FIG. 7 occurs if instead of an integratedharmonic tuner (71) a cascade of two wideband tuners (91, 92) is used,as shown in FIG. 9. The tuning principle remains the same [5], exceptfor some practical imperfections, such as reduced tuning range andmechanical misalignment.

FIG. 10 shows, again in principle the same configuration as in FIG. 8,where, instead of an integrated three-probe tuner, a cascade of atwo-probe harmonic tuner (102) with a single probe wideband tuner (101)is used. Said tuner combination works in a similar way as a three-probetuner (81) or the equivalent, not shown here, which is the use of acascade of three wideband tuners.

Electronic (PIN diode based) tuners are passive tuners; they behavesimilarly to slide-screw electromechanical tuners [7]; they can replacesaid electromechanical tuners arranged in cascade in order to createindependent impedance control; loss of said tuners is higher than ofelectro-mechanical tuners, therefore their employment, despite lowersize and weight, is not favorable; however they do represent atechnically possible alternative.

All mismatch reduction and power saving phenomena described in theprevious sections for one frequency applies in full for each of theother or harmonic frequencies when multi-frequency/harmonic tuners areused which allow pre-matching each said frequency component separately.

As shown in FIG. 11 the requirement for power injection (110) is reducedradically by passive pre-matching (111); in the case of a powertransistor with internal impedance of 1 Ohm (112) the reduction in powerrequirement reaches (ideally) a factor of 12 as shown on axis (113); inreality this number can be higher if insertion losses between injectionamplifiers and DUT are taken into consideration. In terms of commercialcost of associated power amplifiers the ratio is similar.

The actual combination (hybrid) impedance synthesis is shown in FIG. 12;for each of three (harmonic or not) frequencies the total reflectionfactors (121, 122, 123) presented to the DUT port comprise a passivereflection portion (from the tuner) (124, 125, 126) and an activeportion (127, 128, 129), correspondingly; it is obvious that the activeparts (127, 128, 129) are much smaller than the total vectors (121, 122,123) and thus require much less injected power. To be able to generatethis tuning mechanism a passive tuner is required, which is capable ofsynthesizing vectors (124, 125 and 126) independently on each-other [2].

The present invention claims a harmonic active injection load pull setupand an operation method, in which harmonic impedance tuners create apre-match transformation of the high reflection factor required at theoutput of the DUT, at the fundamental and harmonic frequencies, to thenominally 50 Ohm (reflection factor=0) of the power amplifiers used toinject power into the output of the DUT. Obvious alternativeconfigurations of this basic concept are possible but shall not impedeon the originality of the invention.

What is claimed is:
 1. A test setup for power RF active devices (DUT) ata multitude of frequencies F1, F2, . . . FN, comprises a harmonicreceiver/network analyzer, a signal source F1 and a combo RF load;wherein said signal F1 is injected into the input port of said DUT andsaid combo load (reflection factor) is presented to the output port ofsaid DUT; and wherein and the injected and reflected signal waves at theinput and output port of said DUT are sampled by directional couplersand measured by said harmonic receiver; and wherein said combo loadcomprises two components: a) a real or passive component, created by animpedance transformer (tuner), and b) a virtual or active component,created by RF power injection from phase and amplitude modulatedexternal signal sources; said power injection comprising a multitude ofsignal sources at the frequencies F1, F2, . . . FN and said impedancetransformer (tuner) being able to independently tune at each of saidfrequencies.
 2. A test setup as in claim 1, wherein said input signal isat a fundamental frequency Fo and the injected signals at the output areat multiples (harmonic) frequencies FN of said fundamental frequency,with FN=N×Fo with N=1, 2, 3 etc.
 3. A test setup as in claim 1, whereinsaid output signal sources are synchronized (coherent) with the signalsource injected at the input of said DUT.
 4. A method for reducing therequirement for injection power into the DUT output port in a multifrequency load pull test setup, said setup comprising a multitude ofsignal sources at frequencies F1 . . . FN, wherein one signal F1 isinjected into the input port of a DUT and signal sources F2 . . . FN areinjecting power into the output of said DUT; said method comprising animpedance transformer (tuner) which is inserted between the output portof said DUT and the output signal sources, and serving in reducingmismatch and power loss between said DUT output impedance and internalimpedance of said output signal sources.
 5. A method as in claim 4 wheresaid input signal frequency is a fundamental frequency Fo and outputsignal frequencies F2, . . . FN are multiple frequencies (harmonics)N×Fo of said fundamental frequency, wherein N=2, 3 . . . etc.
 6. Amethod as in claim 4, wherein said impedance tuner is a multi-probemulti-frequency tuner, capable of tuning each said frequency (.F2 . . .FN.) separately and independently of each other.
 7. A method as in claim5, wherein said tuner is a multi-probe harmonic tuner, capable of tuningeach harmonic frequency N×Fo independently of each other, wherein N=1,2, 3 etc.
 8. A test setup as in claim 6, wherein said output impedancetuner comprises a cascade of two or more wideband impedance tuners, saidcascade being capable of tuning most or all said frequencies F1 to FNindependently of each other.
 9. A test setup as in claim 6, wherein saidoutput impedance tuner is an electronic tuner.
 10. A test setup as inclaim 6, wherein said output impedance tuner comprises a cascade of twoor more wideband electro-mechanical or electronic tuners, said cascadebeing capable of tuning most or all said frequencies F1 to FNindependently of each other.
 11. A tuning method for hybrid harmonicload pull test systems, said test systems comprising a multitude ofsynchronized signal sources at frequencies (FN=N×Fo, N=1, 2, 3 . . . ),a harmonic receiver, a DUT, an input and an output impedance tuner,signal samplers (couplers) inserted between the DUT and the tuner andfeeding signal to the harmonic receiver and power amplifiers; wherein afundamental frequency (F1=Fo) is injected both into the input and outputof said DUT; and wherein harmonic frequencies (F2, F3 . . . ) areinjected into the output of said DUT; and wherein said impedance tunerpre-matches the conjugate complex internal output impedance of said DUTto the sources at said fundamental (F1) and harmonic frequencies (F2,F3).